This invention relates to distributed amplifiers and relates in particular to Field Emitter Arrays used to form Integrated Distributed Amplifiers.
Distributed amplifiers are shown and described in an Article entitled "Distributed Amplification", by E. L. Ginzton et al., published in the Proceedings of the I.R.E., August 1948, pp. 956-969, and in W. S. Percival's British patent specification No. 460,562 where the basic concepts of distributed amplifiers composed of lumped circuit elements e.g. individual vacuum tubes, are discussed.
Distributed amplifiers, as shown in the Ginzton et al. Article, combine amplifier tubes in a series-like circuit in which tube capacitances (i.e. plate capacitance C.sub.p and grid capacitance C.sub.g) and tube inductances (i.e. plate inductance L.sub.p and grid inductance L.sub.g) may be considered separately. Results are achieved by using the tube capacitances and inductances as lumped circuit elements of a pair of artifical transmission lines in the manner shown in FIG. 1 of the Ginzton et al. Article. Companion transmission lines are formed, with cathode-grid regions of the tubes forming part of one transmission line, and with grid-anode regions of the tubes forming part of another transmission line. These transmission lines are "quasi" transmission lines that only approximate real transmission lines because of the discrete nature of the tubes and the lumped element nature of the impedances interconnecting the tubes.
Ginzton et al. show that ordinary electron tubes in artificial transmission lines make possible wave signal amplification over wide bandwidths. Today, distributed amplifiers typically assume one or two forms: one form of amplifier being based on solid state electron transport, i.e., semiconductors (transistors), and another form of amplifier being based on vacuum electron transport, i.e. vacuum tubes such as the traveling wave tube (TWT). Each form has its own unique limitations.
Power semiconductor devices forming distributed amplifiers are limited in their frequency response by electron saturation drift velocity (about 3.times.10.sup.7 centimeter per second cm/s) and by the inherent capacitance of the channel area of the field effect transistor (FET) or the base area of the bipolar transistor used as the basic component module. Furthermore, because of these two limitations, they cannot be made to have a wide-bandwidth characteristic because of the unavoidable dispersion of, and high value of, electron transit times as electrons transit from their cathodes (e.g., emitters) past one or more grids (e.g., gates) to their anodes (e.g., collectors).
Conventional vacuum tubes (excluding TWTs) suffer from a different set of problems when compared to semiconductor devices. First, spacing of conventional vacuum tube electrodes is too large to allow grid modulation at frequencies much above about 5 gigahertz (ghz), because the electrodes are mechanically "machined" during tube fabrication. Hence, even 1-2 mil grid-cathode spacings obtainable in these structures, prohibits wide-bandwidth as an amplifier characteristic because grid-cathode spacing controls the transit time of electrons flowing from the cathode past the grid(s) to the anode of each vacuum tube in the distributed amplifier. Second, when tubes include an integral grid or grids, Barium (Ba) from the tube's thermionic cathode migrates or evaporates, during tube operation, from the cathode onto insulation layers separating the grids from the Bariated cathode and, as a result, the grids are shorted out. Third, low current densities available from thermionic cathodes employed in vacuum tubes, limited to less than 10 amperes per square centimeter (A/cm.sup.2), do not allow microminiturization of the tubes themselves. Fourth, present vacuum tubes are inherently low reproducibility and high unit cost because they employ hand operations in fabrication.
Accordingly, there is a present need to design and fabricate distributed amplifiers that do not suffer from the problems outlined above. This need is met by this invention which employs for the very first time the technology of integrally gridded Field Emitter Arrays (FEAs), replacing gridded thermionic vacuum tubes and in a form which is susceptible to the same low cost reproducible manufacturing methods as exist in silicon integrated circuit manufacture and processing.
In contrast to the two forms of distributed amplifiers that have just been described, distributed amplifiers utilizing Field Emitter Arrays (FEAs) would offer wide bandwidth capability not attainable with semiconductor devices or with vacuum tubes. Such distributed amplifiers, moreover would be attractive for many technical application where said devices or said tubes can not be readily employed. The present invention is amenable to low cost and reproducible manufacture using existing microelectronics processing techniques. Furthermore, the vacuum aspects of the invention lie well within existing art. Distributed amplifiers although known for five decades, have never been built from Field Emitter Arrays for numerous reasons that would require a lengthly discussion here that would not serve a useful purpose. Primarily because of submicron and sub 1000 Angstrom (A) fabrication technology difficulties and for many other reasons, no one has yet fabricated a Distributed Amplifier made of any Field Emitter Array built by using integrated circuit technology despite the advantages that would accrue to the builder. Furthermore, such devices offer temperature ranges of operation not available to any known semiconductors; those semiconductors which can operate at high temperature, because of wide energy gaps, cannot operate significantly below room temperature because their donor and acceptor states are not shallow compared to kt, where T is the absolute temperature and k is Boltzman's constant.
Henry F. Gray and Richard F. Greene have been continuously active for many years in developing Field Emitter Array Technology and have made significant advances in this technology. They have sought ways to embed Field Emitter Arrays in Integrated Distributed Amplifiers, because there are inherent advantages to be gained through manufacture and utilization of such devices. Their efforts had met with little success until recently. Their success will soon be evidenced by new Distributed Amplifier products that will soon be introduced into the marketplace. Unpublished technical proposals indicating feasibility of demonstration projects involving Distributed Amplifier designs with embedded Field Emitter Arrays have been originated by Gray and Greene.
Gray and Greene have long recognized that it would be desirable to fabricate and provide an ultra-high frequency, wide bandwidth, integrated distributed amplifier based on field emitter arrays, because such amplifiers could be used, inter alia, for medium power amplification of microwaves, millimeter waves, and submillimeter waves. This patent specification represents a project to design and build distributed amplifier modules in accordance with design principles set forth herein.
Henry F. Gray, at the Ballistics Electrons for Transistors Conference held in Santa Barbara, Calif., on 23 March 1987, reviewed Field Emitter Array Electronics and dealt with concepts and applications, and dealt with Current Saturation in n-type Silicon Field Emitter Arrays. Presentations were given on 23 March 1987 by Henry F. Gray that related to the two topics just identified. Sketches of a proposed distributed amplifier were given out at said Conference when the basic concepts of the invention where made available to the public for the very first time. Gray presented these ideas to a technical audience at Hughes (EDD), Torrance, Calif. at their invitation after 23 March 1987.